This invention relates to resonator systems having adaptive optics and more particularly such resonator systems capable of actively generating phase corrections to intracavity radiation to compensate for temporally varying phase perturbations.
An essential requirement of a laser resonator adapted for producing an output beam having high energy is the capability to efficiently extract energy from a gain medium and simultaneously produce a near diffraction limited output beam. For example, in chemical laser systems, mode discrimination by the resonator as well as optical quality of the output beam are often compromised as a result of temporally varying refractive index perturbations within the gain medium, misalignment of the optical elements defining the laser resonator and thermally induced distortions of the optical elements. Since these laser systems are sensitive to misalignment some sort of dynamic alignment control is required.
Feedback systems have been utilized in prior art devices to obtain frequency stabilization of the output beam. As disclosed by Rowley et al in "Nature", London, Vol. 200, page 745, Nov. 23, 1963, typically one of the mirrors defining the optical cavity is dithered to produce a small amplitude modulation of the output beam, a portion of which is directed to a detector. Synchronous mixing of the output of the detector with the dithering signal produces an error signal which cooperates with a servo control mechanism to activate a transducer attached to one of the mirrors defining the optical cavity to change the relative separation of the mirrors defining the cavity to maintain the output beam at a stabilized frequency.
Additionally Forster in U.S. Pat. No. 3,471,803 filed Apr. 28, 1967 discloses a system for stabilizing the frequency of the output beam from a laser resonator. The laser system incorporates a first laser having optical elements capable of being adjusted to tune the laser to a particular frequency and a single mode laser system adapted for producing an output beam having a stable frequency which is utilized as a reference source. Samples of the output beam of both the tunable laser and the stabilized laser are mixed to produce an output signal which is directed to a piezoelectrically driven optical element of the tunable laser. In operation the separation between the optical elements defining the optical cavity of the tunable laser is continuously adjusted to maintain a stabilized output spectrum. See also U.S. Pat. No. 3,471,804.
Essentially all of the prior art devices adapted for producing an output beam having a stabilized frequency control the separation between the mirrors defining the optical cavity to obtain the stabilization. The prior art devices are not adapted for compensating for phase variations in the radiation within the cavity produced by refractive index perturbations within the gain medium. Additionally feedback systems for phase compensation of phase perturbation to intracavity radiation resulting from misalignment of the optical cavity due to thermally induced mirror distortions and/or vibrations are not available. However, O'Meara in U.S. Pat. No. 4,016,451 filed June 24, 1974, discloses an adaptive energy telescope system capable of providing phase shift to the output beam to compensate for phase perturbations to the beam as it passes through an atmosphere to a remote target. This compensation is provided outside the laser resonator. See also U.S. Pat. Nos. 3,731,103, 3,764,213 and 3,727,223.
The present invention provides a means for overcoming the deficiencies associated with presently known high power optical resonators. This is particularly true of the high power annular resonator configurations typically employed in chemical lasers which utilized intracavity axicons. In these resonators temporally varying phase perturbations resulting from imperfections in the axicon surface figure, optical misalignment, thermally induced mirror distortions, and refractive index variations within the gain medium destroy the mode discrimination properties of the resonator thereby degrading the optical beam quality of the output. The present invention provides a means for compensating for such temporal variations by utilizing an actively deformable resonator mirror in conjunction with a closed loop servo system to control the phase distribution of the radiation within the resonator by modifying the surface configuration of the mirror.